Rising rents and massive investments in infrastructure help buoy lower Manhattan. The period includes only two 10-digit sales, but more seen "ready to tee off" in the the second half.

Eight of the 25 largest property sales in the city were in downtown Manhattan during the first half of 2014, compared to only three during the same time a year earlier.

Nearly $9.8 billion changed hands in New York City's largest real estate transactions in the first half of 2014, according to a list of the top 25 deals compiled by Real Capital Analytics for Crain's. That figure was up nearly 9% from the same period a year ago. Office buildings dominated the list, accounting for more than 11 million square feet of space and 17 of the top 25 deals by value, compared to about half of the sales last year.

The geographic distribution of properties changing hands followed a recent trend whereby the action has increasingly shifted away from midtown in favor of downtown, where leasing activity has jumped 30% since the beginning of the year. Eight of the top 25 purchases were downtown, compared to only three in the first six months of 2013. The surge in downtown deals has been spurred by rising rents in the area, which have inspired confidence in the market and an influx in capital, said Dan Fasulo, managing director of Real Capital Analytics.

"Combine that with the near-term completion of a lot of infrastructure, and it makes a very compelling bet for the investment community," said Mr. Fasulo. "On the flip side, as values have improved and more tenants have filled buildings at higher rents, a lot of long-time owners feel they've really dodged a bullet by coming out of the downturn unscathed and are saying maybe it's time to take some chips off the table."

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NYC's top 25 property sales

Data courtesy of Real Capital Analytics Inc. Biggest commercial transactions in New York City in the first half of 2014, ranked by price.

Downtown properties were among the five biggest deals on the list, including the sale of the retail portion of One World Trade Center to Westfield by the Port Authority of New York & New Jersey for $800 million and the purchase of the Citigroup headquarters on Greenwich Street by SL Green for $778 million. On the other hand the two largest deals, and the only two to come in over $1 billion, were in midtown west.

The first half's largest transaction, the sale of 5 Times Square to David Werner in June, ranks as the most expensive since Google bought its property on Eight Avenue in 2010. The $1.5 billion was a victory for seller AVR Realty, which bought the property for $1.3 billion near the peak of the market in 2006 and struggled to pay the property's mortgage during the economic downturn.

"Even 18 months ago, you probably would have bet that AVR was going to be in trouble with that asset," said Mr. Fasulo. "The sale of 5 Times Square is really a function of how much values have come back, because that could have wound up in a distressed scenario had the market not recovered."

A Brooklyn building also made the Top 25 list. One of Chicago's largest landlords purchased a 10-story office building near the Barclay's center for $199 million. The previous owner, GFI Development, bought the property for about $45 million in 2007. The building comes with a strong tenant—the city's Human Resources Administration—and the new owner, American Landmark Properties, expects the area to grow as tech and creative tenants move into downtown Brooklyn. Mr. Fasulo noted that although there is a tremendous amount of real estate activity in Brooklyn, its purchases seldom make the list of the city's largest.

In contrast to the list for largest deals of the first half of 2013, which included five apartment buildings, the 2014 list includes just two—both of which are slated for conversion to condos. The Wellington on the Upper East Side was purchased by from the California Public Employee's Retirement system and GID by O'Connor Capital Partners for $240 million in April, and residents are expected to move into the converted building in 2015. In midtown east, former Gov. Eliot Spitzer sold the Corinthian to Gaia Real Estate holdings for $147 million, and the building's block of apartment buildings are also targeted for conversion. The move toward large condo purchases is a trend that hasn't been seen since 2007, said Mr. Fasulo.

"It's been a while since big apartment buildings were bought for condo conversion," said Mr. Fasulo. "It makes sense when you think about condominium values and how much they've skyrocketed over the past year that this is the marketplace reacting."

Mr. Fasulo cautioned against judging the overall marketplace based on just the first 6 months of transactions. Although this year's list included only two billion-dollar deals, down from last year's three, there are other large deals that are in the pipeline for later in 2014.

"There's only a couple billion-dollar deals, and you would hope at this point in the cycle there would be more, but I know there are more coming," he said. "There are a bunch of billion-dollar deals getting ready to get teed off in Manhattan."